There are many kinds of rail road cars for carrying particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal or other granular materials. These materials are not liquid, yet may in some ways tend to flow in a somewhat liquid-like manner. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, by which the particulate material exits the car under the influence of gravity. Clearly, while the inlet opening need not necessarily have a movable gate (but may include a cover to discourage contamination of the lading or exposure of the lading to the wind), the outlet opening requires a governor of some kind that is movable between a closed position for retaining the lading while the lading is being transported, and an open position for releasing the lading at the destination. The terminology “flow through” or “flow through rail road car” or “center flow” car, or the like, may sometimes be used for cars of this nature where lading is introduced at the top, and flows out at the bottom.
Consider, for example, a hopper car for transporting aggregate, be it gravel or sand. The hopper may have a converging hopper discharge section that has the shape, generally speaking, of an inverted four sided, truncated pyramid. At the bottom of this discharge section is a door, or gate for releasing the aggregate at the destination. On occasion, moist fine aggregate, which may be sand, may have an inconvenient tendency to bridge and stick in the discharge. This may delay the unloading of the car, or require the use of shaking equipment or hammers. It may be helpful to discourage, or break up such bridging in the lading.